Many industries utilize various types of mechanical polishing processes for polishing work pieces. For example, the computer manufacturing industry relies heavily on chemical mechanical polishing (CMP) processes for polishing wafers of ceramics, silicon, glass, quartz, and metals. Such polishing processes generally entail applying the wafer against a rotating pad made from a durable organic substance such as polyurethane. A chemical slurry is utilized that contains a chemical capable of breaking down the wafer substance and an amount of abrasive particles which act to physically erode the wafer surface. The slurry is continually added to the rotating CMP pad, and the dual chemical and mechanical forces exerted on the wafer cause it to be polished in a desired manner.
Of particular importance to the quality of polishing achieved with this method of polishing is the distribution of the abrasive particles throughout the pad. The top of the pad holds the particles by means of fibers or small pores, which provide a friction force sufficient to prevent the particles from being thrown off of the pad due to the centrifugal force exerted by the pad's spinning motion. Therefore, it is important to keep the top of the pad as flexible as possible, to keep the fibers as erect as possible, and to assure that there is an abundance of open pores available to receive newly applied abrasive particles.
One problem that arises with regard to maintaining the pad surface, however, is an accumulation of polishing debris coming from the work piece, the abrasive slurry, and the pad dresser. This accumulation causes a “glazing” or hardening of the top of the pad, mats the fibers down, and thus makes the pad surface less able to hold the abrasive particles of the slurry. These effects significantly decrease the pad's overall polishing performance. Further, with many pads, the pores used to hold the slurry, become clogged, and the overall asperity of the pad's polishing surface becomes depressed and matted. A CMP pad dresser can be used to revive the pad surface by “combing” or “cutting” it. This process is known as “dressing” or “conditioning” the CMP pad. Many types of devices and processes have been used for this purpose. One such device is a disk with a plurality of superhard crystalline particles such as diamond particles attached to a metal-matrix surface.
As semiconductor technology continues toward size reduction to the nano-scale, however, current CMP polishing techniques are proving to be inadequate. With such a reduction in scale, materials utilized to construct circuit elements have become more delicate, both in size and materials. The CMP industry has been required to respond by providing polishing materials and techniques that accommodate these advances. For example, lower CMP polishing pressures, smaller size abrasive particles in the slurry, and polishing pads of a size and nature that do not over polish or damage the wafer must be used. Furthermore, pad dressers that cut asperities in the pad which can accommodate the smaller abrasive particles, and that do not overdress the pad must also be used.
There are a number of problems associated with modifying current CMP processes to accommodate such delicate polishing. With regard to the CMP pad dresser, the superabrasive particles must be significantly smaller than those typically used in currently know dressing operations. Generally speaking, the superabrasive particles are so small that a traditional metal matrix is often unsuitable for holding and retaining them. Further, the smaller size of the superabrasive particles requires that particle tip height be precisely leveled in order to uniformly dress the pad. Traditional CMP pad dressers can have particle tip height variations of more than 50 μm without compromising dressing performance. However, such a variation would render a dresser useless if it were required to dress a CMP pad and achieve polishing of extremely small and delicate circuit elements. In this case, asperities in the dressed pad would have height variations on the same order as the dresser. The highest asperities exert the highest pressure, and would thus scratch and damage the wafer.
In addition to drastic height variations relative to the delicacy of the polishing operation, damage to the wafer can also occur due to the abrasive particles themselves. Sizing of these particles can be problematic, particularly with the smaller sizes required for more delicate polishing operations. Larger abrasive particles that tend to cause surface damage to the wafer are thus difficult to eliminate from the slurry.
Some polishing processes have found it beneficial to add an electrical element to the polishing process which results in electrochemical polishing in conjunction with the mechanical polishing. Such a process is known as electrochemical mechanical polishing (ECMP). In this type of system, conductive materials are removed from a surface to be polished via electrochemical dissolution coupled with mechanical polishing. Because of the electrical element, this method requires less mechanical or forced abrasion. ECMP, therefore, can be used in polishing surfaces that are more susceptible to deforming, breaking and cracking if left to mechanical and/or chemical means alone. Additionally, ECMP can allow for a very fine polish—particularly with such surfaces as copper circuitry.
As a result, polishing tools that are suitable for delicate polishing applications such as those that have arisen with continual reductions in semiconductor size are being sought.